An image sensor is a semiconductor device that converts an optical image to an electric signal. Image sensors are generally classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
In a related art image sensor, a photodiode is formed in a substrate using ion implantation. As the size of a photodiode reduces more and more for the purpose of increasing the number of pixels without an increase in chip size, the area of a light receiving portion reduces, which results in a reduction of image quality.
Also, since a stack height does not reduce as much as the reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion also reduces due to diffraction of light called Airy disk.
As an alternative to overcome this limitation, there is an attempt to minimize a height of an upper surface of the light receiving portion and eliminate light interference due to a metal routing by receiving light through a wafer back side. Image sensors utilizing such an approach can be referred to as back side illumination image sensors.
In a related art back side illumination image sensor, processes for forming a light receiving device and a metal line are performed on a front side of a substrate, and then a back grinding to remove a back side of the substrate by a predetermined thickness is performed. The back grinding is performed so as to adjust an interval between an external module and an optical lens by grinding the back side of the substrate to a proper thickness.
According to a related art back side illumination image sensor, a Silicon On Insulator (SOI) wafer is used as a donor wafer on which a light receiving device and a circuitry are formed. Then, the SOI wafer is bonded to a handle wafer. Thereafter, a backside thinning process for the donor wafer is performed.
The backside thinning process for the donor wafer according to the related art is described below.
First, a backside grinding for the donor wafer is performed such that only a thickness of several tens of micrometers about a top surface of a buried oxide (BOX) remains. Thereafter, an etch-back is performed to complete the backside thinning process.
However, since the related art back side illumination image sensor uses the expensive SOI wafer as the donor wafer, the manufacturing cost is very high.
Also, the backside grinding for the donor wafer according to the related art may cause a wafer edge thinning. If the wafer edge thinning occurs, failures of chips positioned at a wafer edge may be generated, so the yield is lowered.
In addition, according to the related art, since the wafer center is also exposed to plasma damage during the etch-back process to etch the wafer by a thickness of several tens of micrometers, there may be a problem that the degeneration possibility of the image sensor is increased.
Another approach according to the related art is an image sensor (which is referred to as “3D image sensor”) in which a photodiode is formed by depositing amorphous silicon. A readout circuitry is formed on an Si substrate and the photodiode is formed on another wafer, a wafer-to-wafer bonding is performed and thus the photodiode is formed on the readout circuitry. The photodiode and the readout circuitry are connected through a metal line.
After positioning the photodiode on the wafer having the readout circuitry, a bonding process is performed. At this time, the readout circuitry may be not electrically properly connected with the photodiode due to a bonding problem. For example, a metal line is often formed on the readout circuitry and then the wafer-to-wafer-bonding is performed such that the metal line contacts the photodiode. However, at this time, it may be difficult to correctly contact the metal line with the photodiode, and it may be difficult to provide an ohmic contact between the metal line and the photodiode. Also, according to the related art, the metal lines connected with the photodiode may be shorted. Though there are researches for preventing the shortage between the metal lines, it is problematic that a complicated process is required.